1. Field of the Invention
This invention relates generally to color video cameras and, more particularly, to apparatus for extending the lighting range over which a color video camera can operate satisfactorily.
2. Description Relative to the Prior Art
To enhance the usefulness of a color video camera, it is desirable to incorporate therein a capability for existing light operation at low light levels. One straightforward way to extend color camera operation to low light levels is to utilize lenses having high light gathering power. There are, however, practical limits to this approach beyond which camera operation is desirably extended.
It is also known for certain types of video cameras, generally those using imaging tubes, to use special apparatus to increase the effective sensitivity of the imaging device (e.g., see U.S. Pat. No. 3,848,085) and thereby provide for operation over a wider range of light levels. This approach, however, becomes increasingly complex and expensive as continued reductions in the minimum acceptable light level are attempted, particularly for a color camera having plural sensors and the attendant need that such sensors be balanced sensitivity-wise.
A somewhat different approach to extending camera operation utilizes image information from the infrared portion of the spectrum when low light levels are encountered (e.g., see U.S. Pat. No. 3,748,383). This technique recognizes that for many low light level scenes, i.e., from a visible light standpoint, a significant level of infrared is present (e.g., candlelight scenes and firelight scenes).
It would be desirable to extend the range of operation for color cameras by utilizing infrared image information to produce a black-and-white image at very low light levels. A changeover to receive infrared image information should not, however, require a repositioning of the spectrally selective apparatus of the color camera (i.e., the apparatus which separates the image beam into a set of three primary colors), because such repositioning would present serious problems in registering the various color images. Moreover, such changeover is desirably effected automatically and by apparatus adapted to cooperate with semiconductor imaging devices.